Various constant current circuits have been proposed. A constant current circuit disclosed in JP-A-5-35352 has an emitter-follower transistor for generating an output current and a parallel circuit connected to the emitter of the transistor. The parallel circuit comprises a diode and a resistor having a positive temperature coefficient. A current source circuit disclosed in JP-A-2000-124743 has a current mirror constant current circuit serving as a first constant current source circuit as well as second and third constant current source circuits connected between a power supply line and transistors forming the current mirror constant current circuit. A resistor having a negative temperature coefficient is connected between the power supply line and each of transistors composing the second and third constant current source circuits.
A constant current circuit disclosed in JP-A-2002-236521 has a voltage generation circuit and a constant current supply device. The voltage generation circuit has a reference voltage circuit and a temperature characteristic correction circuit connected to the reference voltage circuit as a circuit for correcting the temperature characteristic of the reference voltage circuit. The constant current supply device is a circuit for controlling a current, which is generated by a voltage output by the reference voltage circuit with its temperature characteristic corrected as a current flowing through an internal current detection resistor. The internal current detection resistor has the temperature characteristic opposite to the temperature characteristic of the reference voltage circuit employed in the voltage generation circuit.
A circuit shown in FIG. 12 is proposed as another constant current supply device 1. The constant current supply device 1 controls a main current IL flowing through a main current path 4 to a constant magnitude. The main current path 4 is a path starting from a terminal 2, passing through a resistor R1 for detecting a current flowing through the resistor R1, a MOS transistor Q1 and a resistor R2, and ending at a terminal 3. A voltage appearing between the two ends of the resistor R1 varies in accordance with the main current IL, changing a current flowing through the collector of a transistor Q3 in a current mirror circuit 5, which comprises a transistor Q2, the transistor Q3 and resistors R1, R2 and R3.
A transistor Q4 is connected between the collector of the transistor Q3 and a power supply line 6 linked to a terminal 3. The transistor Q4 and a transistor Q5 form a current mirror circuit 7. A self-bias constant current circuit 9 using a voltage appearing between the base and the emitter as a reference voltage is connected between the transistor Q2 and the power supply line 6. On the other hand, a self-bias constant current circuit 10 using a voltage appearing between the base and the emitter as a reference voltage is connected between the transistor Q5 and a power supply line 8.
In this constant current supply device 1, when the main current IL exceeds a target current magnitude, for example, the voltage appearing between the two ends of the resistor R1 also exceeds a predetermined level, increasing a voltage appearing between the base and emitter of the transistor Q3. Thus, a current flowing through the collector of the transistor Q3 also rises as well. As a result, the electric potential appearing at the gate of the transistor Q1 decreases, causing feedback control to operate to result in a decrease in the main current IL.
In this constant current supply device 1, however, when the current output by the constant current circuit 9 changes due to, among other causes, a change in temperature, the change in the output current causes currents flowing through the transistors Q2, Q3, Q4 and Q5 to change as well. As a result, the electric potential appearing at the gate of the transistor Q1 also changes, causing the main current IL to deviate from the target current magnitude.
In this case, when the constant current circuits 9 and 10 are in a state of being associated with each other by a current mirror circuit not shown in the figure, the current flowing through the transistor Q5 exhibits the same trend of changes caused by changes in temperature as the current output by the constant current circuit 10. Thus, the magnitude of the change in main current IL is reduced. In actuality, however, the resistances of the resistors R1 to R3 also change. Therefore, when the operating temperature changes from −40 degrees Celsius to 145 degrees Celsius, the main current IL changes by about 200 mA from a target current magnitude of 1.5 A as shown in FIG. 2B. In this case, by designing each of the constant current circuits 9 and 10 into a circuit configuration using a band gap reference, the magnitude of the change can be reduced. However, the circuit configuration becomes complicated.
In the case of another typical constant current circuit using a constant current as a bias current, a current output by the circuit is detected as a voltage appearing between the two ends of a resistor, and changes of the detected voltage are fed back as changes of a voltage appearing between the base and emitter of a transistor. This constant current is generated on the basis of a reference voltage generated by a reference voltage generation circuit.
A reference voltage generation circuit disclosed in JP-A-2000-112548 generates a high precision reference output voltage slightly lower than the electric potential of the power supply by fine adjustment of the resistance of a resistor device in a laser trimming process. In JP-A-2002-091589, after IC resin encapsulation, trimming adjustment of a resistor can be carried out to optimize the temperature characteristic of the reference voltage.
In addition, as shown in FIG. 13, an IC 201 comprising a plurality of constant current output circuits 202 has also been proposed. The constant current output circuits 202 each operate by receiving a battery voltage VMAIN supplied by power supply lines 203 and 204. In spite of variations in the battery voltage VMAIN, the IC 201 outputs a constant current to every load RL connected to one of terminals 205 of the IC 201. Each of the constant current output circuits 202 comprises transistors Q201 to Q205, resistors R201 to R205 as well as constant current circuits 206 and 207. The resistor R201 is a current detection resistor and the transistor Q203 is an output transistor.
When the current flowing through a load RL connected to the constant current output circuit 202 exceeds a set target magnitude, a voltage appearing between the two ends of the resistor R201 increases and, hence, the current flowing through the collector of the transistor Q202 increase while a voltage appearing between the gate and source of the transistor Q203 decreases. As a result, the current flowing through the drain of the transistor Q203 decreases, exhibiting an effect of restoring the current flowing through the load RL to the set target magnitude.
When an A1 shunt resistor and an LDMOS transistor are used respectively as the resistor R201 and the transistor Q203 in the constant current output circuit 202, the temperature coefficient of the resistor R201 is about equal to that of the transistor Q203. Thus, changes in current, which are caused by changes in temperature, are compensated for to a certain degree. In order to obtain a high precision constant current output characteristic, coordinated current adjustment needs to be performed by carrying out adjustment works such as a laser trimming process for the resistor R204 and a trimming process for the transistors Q201 and Q202 for every constant current output circuit 202.
Except for a case in which a constant current output circuit 202 is embedded in the IC 201 as 1 channel, as the number of channels rises, the size of a circuit for trimming process use and the time it takes to carry out the trimming process increases as well. As a result, the manufacturing cost also rises.